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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Driving ultrafast spin and energy modulation in quantum well states via photo-induced electric fields

Abstract The future of modern optoelectronics and spintronic devices relies on our ability to control the spin and charge degrees of freedom at ultrafast timescales. Rashba spin-split quantum well states, 2D states that develop at the surface of strong spin-orbit coupling materials, are ideal given the tunability of their energy and spin states. So far, however, most studies have only demonstrated such control in a static way. In this study, we demonstrate control of the spin and energy degrees of freedom of surface quantum well states on Bi 2 Se 3 at picosecond timescales. By means of a focused laser pulse, we modulate the band-bending, producing picosecond time-varying electric fields at the material’s surface, thereby reversibly modulating the quantum well spectrum and Rashba effect. Moreover, we uncover a dynamic quasi-Fermi level, dependent on the Lifshitz transition of the second quantum well band bottom. These results open a pathway for light-driven spintronic devices with ultrafast switching of electronic phases, and offer the interesting prospect to extend this ultrafast photo-gating technique to a broader host of 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Prospects for metal halide perovskite-based tandem solar cells

Over the past decade, metal halide perovskite photovoltaics have been a major focus of research, with single-junction perovskite solar cells evolving from an initial power conversion efficiency of 3.8% to reach 25.5%. The broad bandgap tunability of perovskites makes them versatile candidates as the subcell in a tandem photovoltaics architecture. Stacking photovoltaic absorbers with cascaded bandgaps in a multi-junction device can potentially overcome the Shockley–Queisser efficiency limit of 33.7% for single-junction solar cells. There is now intense activity in developing tandem solar cells that pair perovskite with either itself or with a variety of mature photovoltaic technologies such as silicon and Cu(In,Ga)(S,Se) 2 (CIGS). In this review, we survey recent advances in the field and discuss its outlook.

14 SOLAR ENERGY↗
DOE OSTI2020

Gene-environment interactions due to quantile-specific heritability of triglyceride and VLDL concentrations

"Quantile-dependent expressivity" is a dependence of genetic effects on whether the phenotype (e.g., triglycerides) is high or low relative to its distribution in the population. Quantile-specific offspring-parent regression slopes (β OP ) were estimated by quantile regression for 6227 offspring-parent pairs. Quantile-specific heritability (h 2 ), estimated by 2β OP /(1 + $\Gamma$ spouse ), decreased 0.0047 ± 0.0007 (P = 2.9 × 10 -14 ) for each one-percent decrement in fasting triglyceride concentrations, i.e., h 2 ± SE were: 0.428 ± 0.059, 0.230 ± 0.030, 0.111 ± 0.015, 0.050 ± 0.016, and 0.033 ± 0.010 at the 90th, 75th, 50th, 25th, and 10th percentiles of the triglyceride distribution, respectively. Consistent with quantile-dependent expressivity, 11 drug studies report smaller genotype differences at lower (post-treatment) than higher (pre-treatment) triglyceride concentrations. This meant genotype-specific triglyceride changes could not move in parallel when triglycerides were decreased pharmacologically, so that subtracting pre-treatment from post-treatment triglyceride levels necessarily created a greater triglyceride decrease for the genotype with a higher pre-treatment value (purported precision-medicine genetic markers). In addition, sixty-five purported gene-environment interactions were found to be potentially attributable to triglyceride's quantile-dependent expressivity, including gene-adiposity (APOA5, APOB, APOE, GCKR, IRS-1, LPL, MTHFR, PCSK9, PNPLA3, PPARγ2), gene-exercise (APOA1, APOA2, LPL), gene-diet (APOA5, APOE, INSIG2, LPL, MYB, NXPH1, PER2, TNFA), gene-alcohol (ALDH2, APOA5, APOC3, CETP, LPL), gene-smoking (APOC3, CYBA, LPL, USF1), gene-pregnancy (LPL), and gene-insulin resistance interactions (APOE, LPL).

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Spiers Memorial Lecture: Next generation chalcogenide-based absorbers for thin-film solar cells

Inorganic-based thin-film photovoltaics (TFPV) represents an important component of the growing low-carbon energy market and plays a vital role in the drive toward lower cost and increased penetration of solar energy. Yet, commercialized thin-film absorber technologies suffer from some non-ideal characteristics, such as toxic or non-abundant element use (e.g., CdTe and Cu(In,Ga)(S,Se) 2 , which bring into question their suitability for terawatt deployment. Numerous promising chalcogenide, halide, pnictide and oxide semiconductors are being pursued to bridge these concerns for TFPV and several promising paths have emerged, both as prospective replacements for the entrenched technologies, and to serve as partner (i.e., higher bandgap) absorbers for tandem junction devices—e.g., to be used with a lower bandgap Si bottom cell. The current perspective will primarily focus on emerging chalcogenide-based technologies and provide both an overview of absorber candidates that have been of recent interest and a deeper dive into an exemplary Cu 2 BaSnS 4 -related family. Altogether, considering the combined needs of high-performance, low-cost, and operational stability, as well as the experiences gained from existing commercialized thin-film absorber technologies, chalcogenide-based semiconductors represent a promising direction for future PV development and also serve to highlight common themes and needs among the broader TFPV materials family.

14 SOLAR ENERGY↗
DOE OSTI2021

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators.

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi 2 Se 3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. Additionally, by applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (similar to 6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Low-damage electron beam lithography for nanostructures on Bi 2 Te 3 -class topological insulator thin films

Nanostructured topological insulators (TIs) have the potential to impact a wide array of condensed matter physics topics, ranging from Majorana physics to spintronics. However, the most common TI materials, the Bi 2 Se 3 family, are easily damaged during nanofabrication of devices. In this paper, we show that electron beam lithography performed with a 30 or 50 kV accelerating voltage—common for nanopatterning in academic facilities—damages both nonmagnetic TIs and their magnetically doped counterparts at unacceptable levels. We additionally demonstrate that electron beam lithography with a 10 kV accelerating voltage produces minimal damage detectable through low-temperature electronic transport. In conclusion, although reduced accelerating voltages present challenges in creating fine features, we show that with careful choice of processing parameters, particularly the resist, 100 nm features are reliably achievable.

42 ENGINEERING↗
DOE OSTI2021

Asymmetric Fraunhofer spectra in a topological insulator-based Josephson junction

Josephson junctions with topological insulators as their weak link (S–TI–S junctions) are predicted to host Majorana fermions, which are key to creating qubits for topologically protected quantum computing. But the details of the S–TI–S current-phase relation and its interplay with magnetic fields are not well understood. We fabricate a Bi 2 Se 3 junction with NbTi leads and measure the Fraunhofer patterns of the junction with applied in-plane fields. We observe that asymmetric Fraunhofer patterns appear in the resistance maps of B z vs B x,y , with aperiodic node spacings. These asymmetric patterns appear even at zero parallel field and for temperatures up to 1 K. The anomalous features are compared to asymmetric Fraunhofer patterns expected for finite Cooper pair momentum shifts as well as geometric effects. We show that the geometric effects can dominate, independent of in-plane field magnitude. Furthermore, these results are important for differentiating geometrical phase shifts from those caused by Cooper pair momentum shifting, Majorana mode signatures, or other unconventional superconducting behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Reexamining circular dichroism in photoemission from a topological insulator

The orbital angular momentum (OAM) of electron states is an essential ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital-angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been assumed to be a direct probe of OAM and, by proxy, of the Berry curvature of electronic bands in energy- and momentum-space. Indeed, topological surface states have been shown to exhibit angle-dependent CD (CDAD), and more broadly, CD is often interpreted as evidence of spin-orbit coupling. Meanwhile, it is well-established that CD originates from the photoemission matrix elements, which can have extrinsic contributions related to the experimental geometry and the inherently broken inversion symmetry at the sample surface. Therefore, it is important to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator Bi 2 ⁢Se 3 over a wide range of incident photon energies. Not only do we observe angle-dependent CD in the surface states, as expected, but we also find CD of a similar magnitude in virtually all bulk bands. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that “hidden” OAM—localized to atomic sites within each unit cell—contributes significantly. Additional effects, including inter-atomic interference and final-state resonances, are responsible for the rapid variation of the CDAD signal with photon energy.

Sidilkover, Ittai [Tel Aviv University, Tel Aviv (↗
DOE OSTI2021

Strong-field physics in three-dimensional topological insulators

We investigate theoretically the strong-field regime of light-matter interactions in the topological-insulator class of quantum materials. In particular, we focus on the process of nonperturbative high-order harmonic generation from the paradigmatic three-dimensional topological insulator bismuth selenide (Bi 2 Se 3 ) subjected to intense midinfrared laser fields. We analyze the contributions from the spin-orbit-coupled bulk states and the topological surface bands separately and reveal a major difference in how their harmonic yields depend on the ellipticity of the laser field. Furthermore, bulk harmonics show a monotonic decrease in their yield as the ellipticity increases, in a manner reminiscent of high harmonic generation in gaseous media. However, the surface contribution exhibits a highly nontrivial dependence, culminating with a maximum for circularly polarized fields. We attribute the observed anomalous behavior to (i) the enhanced amplitude and the circular pattern of the interband dipole and the Berry connections in the vicinity of the Dirac point and (ii) the influence of the higher-order, hexagonal warping terms in the Hamiltonian, which are responsible for the hexagonal deformation of the energy surface at higher momenta. The latter are associated directly with spin-orbit-coupling parameters. Our results thus establish the sensitivity of strong-field-driven high harmonic emission to the topology of the band structure as well as to the manifestations of spin-orbit interaction.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Control of magnetic properties of MnBi 2 Te 4 using a van der Waals ferroelectric III 2 -VI 3 film and biaxial strain

Using the first-principles calculations, we systematically investigated magnetic properties of a MnBi 2 Te 4 (MBT) monolayer in contact with III 2 -VI 3 two-dimensional ferroelectric substrates. As the electric polarization in the ferroelectric films is switched, a semiconductor to half-metal transition occurs in MBT, and its magnetic easy axis in several cases reorients. As an example, the Curie temperature of MBT/In 2 Se 3 can be enhanced to 21.5 or 32 K under the influence of electric polarization and biaxial strain, much higher than that of the pristine MBT monolayer, which is 14.5 K in our calculations. Our results in this work suggest effective ways to tune magnetic properties of MBT and provide useful insights for the further development of these important van der Waals functional materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Majorana bound states in a d -wave superconductor planar Josephson junction

We study phase-controlled planar Josephson junctions comprising a two-dimensional electron gas with strong spin-orbit coupling and d-wave superconductors, which have an advantage of a high critical temperature. We show that a region between the two superconductors can be tuned into a topological state by the in-plane Zeeman field, and can host Majorana bound states. The phase diagram as a function of the Zeeman field, chemical potential, and the phase difference between superconductors exhibits the appearance of Majorana bound states for a wide range of parameters. We further investigate the behavior of the topological gap and its dependence on the type of d-wave pairing, i.e., d, d+is, or d+id', and note the difficulties that can arise due to the presence of gapless excitations in pure d-wave superconductors. On the other hand, the planar Josephson junctions based on superconductors with d+is and d+id' pairings can potentially lead to realizations of Majorana bound states. Furthermore, our proposal can be realized in cuprate superconductors, e.g., in a twisted bilayer, combined with the layered semiconductor Bi 2 O 2 Se.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Changes of Magnetism in a Magnetic Insulator due to Proximity to a Topological Insulator

We report here the modification of magnetism in a magnetic insulator Y 3 Fe 5 O 12 thin film by topological surface states (TSS) in an adjacent topological insulator Bi 2 Se 3 thin film. Ferromagnetic resonance measurements show that the TSS in Bi2Se3 produces a perpendicular magnetic anisotropy, results in a decrease in the gyromagnetic ratio, and enhances the damping in Y 3 Fe 5 O 12 . Such TSS-induced changes become more pronounced as the temperature decreases from 300 to 50 K. These results suggest a completely new approach for control of magnetism in magnetic thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Line-Graph Approach to Spiral Spin Liquids

Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by subdimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is nonbipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases. As examples, we examine neutron scattering experiments performed on two spinel compounds, ZnCr 2 Se 4 and CuInCr 4 Se 8 , to demonstrate the feasibility of this new approach and expose its possible limitations in experimental realizations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Plasmon coupling in topological insulator multilayers

Topological insulators (TIs) house two-dimensional Dirac plasmons on their surfaces. In TI thin films, plasmons on the top and bottom surfaces couple electrostatically, giving rise to an acoustic and an optical plasmon mode. By extension, a superlattice comprising TI layers and trivially-insulating layers could house multiple complex plasmon modes which could be used to create a new type of Dirac metamaterial. In this paper, we synthesize TI superlattices, fabricate them into stripe arrays, characterize their optical and plasmonic properties, and model the samples using transfer matrices. Furthermore, we excite plasmon modes that couple to the phonons in the superlattice, resulting in hybrid plasmon phonon polaritons. These modes are modeled using an analytical Fano resonance model and the extracted resonant positions are reproduced by transfer matrix modeling. We also excite an epsilon-near-zero mode in the top dielectric material (Bi 0.5 In 0.5 ) 2 Se 3 . Understanding the behavior of the polariton modes in this complex system will lend insight into the many-body interaction in low-dimensional systems.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Linearly dispersive bands at the onset of correlations in K 𝑥 ⁢C 60 films

Molecular crystals are a flexible platform to induce novel electronic phases. Due to the weak forces between molecules, intermolecular distances can be varied over larger ranges than interatomic distances in atomic crystals. On the other hand, the hopping terms are generally small, which results in narrow bands, strong correlations, and heavy electrons. Here, by growing K x C 60 fullerides on hexagonal layered Bi 2 Se 3 , we show that upon doping the series undergoes a Mott transition from a molecular insulator to a correlated metal and an in-gap state evolves into highly dispersive Dirac-like fermions at half filling, where superconductivity occurs. This picture challenges the commonly accepted description of the low-energy quasiparticles as appearing from a gradual electron doping of the conduction states and suggests an intriguing parallel with the more famous family of the cuprate superconductors. More in general, it indicates that molecular crystals offer a viable route to engineer electron-electron interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Improved VOC in RbF-Treated Cu(In,Ga)Se2 Solar Cells via Passivation of Recombination Centers

Cu(In,Ga)Se 2 (CIGS) solar cells have benefited in recent years from the addition of heavy alkali elements, such as Rb, which increase the solar cell open-circuit voltage ( V OC ). To investigate the source of this improvement, here, we compare samples with and without Rb to perform a quantitative comparison of electronic defects and minority carrier lifetime. Deep-level transient and optical spectroscopy measurements were performed on two sets of rubidium fluoride (RbF)-treated and untreated CIGS, and three distinct traps were identified regardless of RbF treatment. The RbF treatment was found to reduce the concentration of the H2 trap, which was previously found to act as a recombination center and is located preferentially at CIGS grain boundaries. Time-resolved photoluminescence measurements showed an increase in effective lifetime after RbF and nearly all lifetime improvement resulted from reductions in bulk recombination. The observed V OC improvement is well correlated with increased minority carrier lifetime and acceptor concentration, which led to increases and decreases in electron and hole quasi-Fermi levels, respectively.

Cu(In Ga)Se2 (CIGS)↗
DOE OSTI2020

In-situ Microscopy Characterization of Cu(In,Ga)Se2 Potential-Induced Degradation

We report on the role of sodium in potentialinduced degradation (PID) of Cu(In, Ga)Se 2 (CIGS) solar cells. In-situ microscopy characterizations on AFM platform were performed on two stressed CIGS device under room temperature (RT) and high temperature (HT) at 85 degrees C. During PID stressing we observed depletion region gets wider as Na migrates, p-n junction becomes leaky at RT for over a month; and similar junction evolution was observed for HT-stressed sample, eventually the junction collapsed after 18 hours. The diode behaviors were confirmed by dark I-V measurement. Time-of Flight secondary-ion mass spectrometry reveals that the Na accumulates on ZnO and CdS side, as well as the upper layer of CIGS layer. The results indicate that Na drifted by the voltage applied on the soda-lime glass, then diffuse through the whole device. And the sodium profiles have different points of evolution due to the temperature differences between the two stressed samples. The consistent results unambiguously show how Na from substrate glass causes PID in CIGS solar cells.

14 SOLAR ENERGY↗
DOE OSTI2020

The Effects of Absorber Thickness on Reverse-Bias Damage in Cu(In,Ga)Se2 Solar Cells

We studied a variety of absorber thicknesses in Cu(In,Ga)Se 2 devices to examine the ability of the cell to withstand reverse current flow without damage. Absorbers were varied from 2.5 um to 0.4 um, with the expectation that larger reverse current would be allowed to flow at lower voltages as absorbers were made thinner. Our initial experiments showed promise. However, as the efficiency of thin devices was improved, the reverse current allowed at a given voltage decreased. Here we present our negative results on the ability of thin devices to allow higher reverse currents at lower voltages. We also give details on our procedures for reverse-bias testing small-area solar cells. We hope this information will be useful for those who want to study reverse-bias at the cell level.

14 SOLAR ENERGY↗